Is reentry critical for visual awareness of object presence?

Failure to detect meaning in RSVP at 27 ms per picture

The human visual system has the remarkable ability to rapidly detect meaning from visual stimuli. Potter, Wyble, Hagmann, and McCourt (Attention, Perception, & Psychophysics, 76, 270-279, 2014) tested the minimum viewing time required to obtain meaning from a stream of pictures shown in a rapid serial visual presentation (RSVP) sequence containing either six or 12 pictures. They reported that observers could detect the presence of a target picture specified by name (e.g., smiling couple) even when the pictures in the sequence were presented for just 13 ms each. Potter et al. claimed that this was insufficient time for feedback processing to occur, so feedforward processing alone must be able to generate conscious awareness of the target pictures. A potential confound in their study is that the pictures in the RSVP sequence sometime contained areas with no high-contrast edges, and so may not have adequately masked each other. Consequently, iconic memories of portions of the target pictures may have persisted in the visual system, thereby increasing the effective presentation time. Our study addressed this issue by redoing the Potter et al. study, but using four different types of masks. We found that when adequate masking was used, no evidence emerged that observers could detect the presence of a specific target picture, even when each picture in the RSVP sequence was presented for 27 ms. On the basis of these findings, we cannot rule out the possibility that feedback processing is necessary for individual pictures to be recognized.

Conscious Vision Proceeds from Global to Local Content in Goal-Directed Tasks and Spontaneous Vision

The reverse hierarchy theory (Hochstein and Ahissar, 2002) makes strong, but so far untested, predictions on conscious vision. In this theory, local details encoded in lower-order visual areas are unconsciously processed before being automatically and rapidly combined into global information in higher-order visual areas, where conscious percepts emerge. Contingent on current goals, local details can afterward be consciously retrieved. This model therefore predicts that (1) global information is perceived faster than local details, (2) global information is computed regardless of task demands during early visual processing, and (3) spontaneous vision is dominated by global percepts. We designed novel textured stimuli that are, as opposed to the classic Navon's letters, truly hierarchical (i.e., where global information is solely defined by local information but where local and global orientations can still be manipulated separately). In line with the predictions, observers were systematically faster reporting global than local properties of those stimuli. Second, global information could be decoded from magneto-encephalographic data during early visual processing regardless of task demands. Last, spontaneous subjective reports were dominated by global information and the frequency and speed of spontaneous global perception correlated with the accuracy and speed in the global task. No such correlation was observed for local information. We therefore show that information at different levels of the visual hierarchy is not equally likely to become conscious; rather, conscious percepts emerge preferentially at a global level. We further show that spontaneous reports can be reliable and are tightly linked to objective performance at the global level.

SIGNIFICANCE STATEMENT

Is information encoded at different levels of the visual system (local details in low-level areas vs global shapes in high-level areas) equally likely to become conscious? We designed new hierarchical stimuli and provide the first empirical evidence based on behavioral and MEG data that global information encoded at high levels of the visual hierarchy dominates perception. This result held both in the presence and in the absence of task demands. The preferential emergence of percepts at high levels can account for two properties of conscious vision, namely, the dominance of global percepts and the feeling of visual richness reported independently of the perception of local details.

Early visual processing allows for selective behavior, shifts of attention, and conscious visual experience in spite of masking

Object-substitution masking (OSM) occurs when a briefly displayed target in a search array is surrounded by a mask, which remains onscreen after the target has disappeared. It has been suggested that OSM results from a specific interference with reentrant visual processing, while the initial feedforward processing is left intact. Here, we tested the prediction that the fastest saccadic responses towards a masked target, supposedly triggered before the onset of reentrant processing, are not impaired by OSM. Indeed, saccades faster than 350ms "escaped" the influence of the mask. Notably, participants' judgements of subjective awareness indicated that stimulus processing during this early stage is not entirely devoid of conscious awareness. Furthermore, the N2pc event-related potential component indicated shifts of spatial attention towards the masked targets on trials with correct fast saccades, suggesting that both target detection and spatial attention can be based on the computations accomplished during the initial feedforward sweep.

Top-down preparation modulates visual categorization but not subjective awareness of objects presented in natural backgrounds

Top-down processes are widely assumed to be essential in visual awareness, subjective experience of seeing. However, previous studies have not tried to separate directly the roles of different types of top-down influences in visual awareness. We studied the effects of top-down preparation and object substitution masking (OSM) on visual awareness during categorization of objects presented in natural scene backgrounds. The results showed that preparation facilitated categorization but did not influence visual awareness. OSM reduced visual awareness and impaired categorization. The dissociations between the effects of preparation and OSM on visual awareness and on categorization imply that they influence at different stages of cognitive processing. We propose that preparation influences at the top of the visual hierarchy, whereas OSM interferes with processes occurring at lower levels of the hierarchy. These lower level processes play an essential role in visual awareness.

The fastest saccadic responses escape visual masking

Object-substitution masking (OSM) occurs when a briefly presented target in a search array is surrounded by small dots that remain visible after the target disappears. The reduction of target visibility occurring after OSM has been suggested to result from a specific interference with reentrant visual processing while the initial feedforward processing is thought to be left intact. We tested a prediction derived from this hypothesis: the fastest responses, being triggered before the beginning of reentrant processing, should escape the OSM interference. In a saccadic choice reaction time task, which gives access to very early stages of visual processing, target visibility was reduced either by OSM, conventional backward masking, or low stimulus contrast. A general reduction of performance was observed in all three conditions. However, the fastest saccades did not show any sign of interference under either OSM or backward masking, as they did under the low-contrast condition. This finding supports the hypothesis that masking interferes mostly with reentrant processing at later stages, while leaving early feedforward processing largely intact.